1. Field of the Invention
The present invention relates to an automatic focus adjusting system of a microscope employed in a surface inspection apparatus.
2. Prior Art
Various automatic focus adjusting systems employed in optical image formation apparatuses such as a camera, a microscope, and the like have been proposed. These proposals are classified into three kinds. One of them uses trigonometrical survey system. This system is unsuitable for the microscope. The second one is to analyze a video signal converted from an optical image. Various suggestions have been made with respect to methods for analyzing the video signal. The third one is to form two images of an object having a parallax.
Except the trignometrical survey system, the other automatic focus adjusting systems described above can be utilized on condition that a video signal having a contrast is obtained. Therefore, when the surface of a plain and blank sheet is inspected by a microscope, these systems are not effective unless lights are projected on the sheet to form a pattern in shades thereon so that the pattern is used as a target to be observed in an automatic focus adjustment. The sheet is moved under the microscope to inspect its surface. At this time, if the surface of the sheet is not fluctuated in accordance with the movement of the sheet, a focusing for an inspection can be maintained by giving a mark at a position having no influence in inspecting the sheet surface, and by bringing the mark into focus. When the sheet surface is irregular, focusing operations are required to be made each time the sheet moves. If a sheet is macroscopically flat, its horizontal movement macroscopically gives no appearance of fluctuation in the case of a microscope having a low magnification. If a microscope having a high magnification is used to inspect such a macroscopically flat sheet, even a slight irregularity of the sheet surface corresponds to a serious vertical fluctuation of the surface of the sheet under the inspection, so that the surface under the inspection becomes out of focus. This is because the focal depth of such a high magnification microscope is short and the visual field thereof is narrow. Therefore, it is necessary to make focus adjusting operations each time the sheet moves. In addition, the above conventional method for making an automatic focus adjustment is not applicable to the inspection of a plain and blank sheet surface.